Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface, showing a first image in a sequence of a first embodiment of the new design;

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof;

FIG. 4 is a front view of a display panel or portion thereof with a transitional graphical user interface, showing a first image in a sequence of a second embodiment of the new design;

FIG. 5 is a second image thereof; and,

FIG. 6 is a third image thereof.

The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1-3 and FIGS. 4-6. The process or period in which one image transitions to another image forms no part of the claimed design.

The broken line portions of the display screen and graphical user interface elements form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 